The present invention relates to ultrasonic imaging system probes, and, more particularly, to a novel ultrasound probe having increased rejection of undesired common-mode ("noise") signals.
Ultrasonic array imagers form viewable display images from a coherent summation of many individual channel signals. The instantaneous dynamic range of each of these signals often exceeds 60dB For example, a very high instantaneous dynamic range may be required for processing the reflected ultrasonic signal from a blood cell, compared to the reflected signal from calcified plaque in a blood vessel. Because the coherent sum must be accurately performed, even with high instantaneous dynamic signal range, individual signals must be combined with accurate time/phase delay so that both the maxima and minima (nulls) are faithfully produced If there is any common-mode "noise" (i.e. undesired signal) present, the system dynamic range will be reduced, due to the degradation of the coherent sum maxima and the more serious degradation of minima (where destructive interference should occur) so that the clinical utility of the resulting image is reduced. Typically, such common-mode noise can be found in ultrasonic systems due to electromagnetic pickup of extraneous signals from the environment. In fact, it is often the ultrasonic imaging system which itself creates an environment with a significant amount of radiated energy for common-mode pickup, due to various system portions (such as local oscillators, signal processing circuits, video display circuits and the like) radiating energy in the ultrasonic signal bandwidth. While efficient shielding is desirable, it is often difficult to achieve, so that there is always some amount of common-mode electromagnetic noise pickup. Often this noise is picked up through the ultrasonic transducer probe which, having many relatively high impedance (typically greater than 100 ohm) transducer elements each coupled to a long (6-10 feet) cable of somewhat lower impedance (typically less than 80 ohms), allows each probe to act as an antenna for electromagnetic signals. Since the personnel utilizing the ultrasonic imaging system prefer a probe having relatively low mass and highly flexible cables, good electromagnetic shielding of the probe/cable assembly is difficult to implement.
While it has been proposed to alternate signal phase in the ultrasonic receiver electronics channels, in order to minimize common-mode noise, the typical implementation thereof utilizes an amplifier, with either a differential input or a differential output, so that signals from adjacent channels are connected to opposite sides of the differential devices. One such system is described and claimed in our co-pending U.S. Pat. application Ser. No. 07/445,521, filed Dec. 4, 1989, now U.S. Pat. No. 4,984,465 issued Jan. 15, 1991, assigned to the assignees of the present invention, and incorporated herein in its entirety by reference. Such a system, using a transformer in each channel, fails to suppress common-mode noise pickup ahead of the transformer. It is highly desirable to provide an ultrasonic imaging system probe having undesired common-mode signal rejection in the entirety of each imaging system channel.